A 3.6 FD is std in an ABF box, you can put a 3.9 or a 4 in which will gear down the output of the box in that it will turn .3 or .4 times more per revolution.
The std 8v box has the same FD ratio but is geared a bit lower through the range....
Are you sure you did not have a leaking seal when the clutch started slipping? it only takes a drip or two of oil to get into the friction material to make it slip at peak torque.
I have run a modded KR before with no slip, granted it does not have the torque of a turbo which is a big factor but you should just be lighting up the wheels rather than slipping

AS DH says the later boxes are designed to give economy / higher top speed but with a hike in torque that a map/more boost will give you then a longer box makes more sense as you can use the torque to more effect.